[Property Editor] Make it clear if a value is expression, or if it's null because it's optional

[elliette]

Currently, we don't display expressions differently than literal values. So a user might see a string input with title and not know if title is the variable name or the literal string.

Similarly, an input might say null, but that could mean the default value is null or that its optional and there is no value.

It would be nice to figure out how to distinguish between these cases in the UI.

[kenzieschmoll]

Can we use a fixed font style text and maybe a different color for expressions? Similar to how when backticks are added in google docs, the color and font changes to show that code is being referenced.